1. Field of the Invention
The present invention relates to conduct and organization of land seismic surveys where multiple swept frequency vibratory sources are operating in various sectors of an area of interest, and to processing of seismic data obtained in such surveys.
2. Description of the Related Art
To achieve a high-resolution land seismic image from surface to the deepest target of interest requires a surface sampling grid that records unaliased elastic waves. If we consider a 5 m spatial sampling grid with a 6000 m aperture, this would require approximately 600,000 active channels. This receiver configuration is currently not economic or practical. This limitation can be overcome by shooting outside a maximum channel constrained receiver spread (10,000 to 100,000 channels) with multiple vibroseis fleets at the expense of increasing the total acquisition time by the vibration point (VP) repetition factor. Using new vibrator source control technology, the total acquisition time can be significantly reduced using new high-productivity vibroseis methods. The most productive method is what is known as the independent simultaneous sweep. One available service according to the independent simultaneous sweep method is that provided under the trademark ISS® of BP p.l.c. of the U. K. There is, however, a risk of interference between vibroseis sources when their initiation times are nearly the same, or what is known as near-simultaneous sweeps. High productivity vibroseis survey methods also offer the risk during the survey of interference between signals from vibroseis trucks when they are positioned close to one another.
During the survey preplan process, land fleets, also known as survey crews, are pre-assigned GPS coordinates and during operations sweep independently without knowledge of the surrounding fleet positions. The seismic observer in the recording truck can visually see the position of all fleet positions and use radio communication to optimize movement of all fleets. This is significant in areas with many obstructions. It is not practical to pre-survey every vibration point for high productivity surveys because of their close spacing, such as in a 25 m grid. A survey crew member known as a vibrator pusher is located in a survey vehicle and helps guide the fleet in the field. Both the vibrator driver and pusher have a GPS monitor which helps guide them from one VP to the next. Radio communication with the observer helps to zoom into digital terrain maps and provide on the ground guidance. If the vibroseis fleets are not separated by a large distance, the high energy surface wave and signal crosstalk interference mask the reflection signals.
Crosstalk occurs when there are other sources firing at different times and the energy arrives during the listen time for a vibroseis source. For the Independent Simultaneous Sources (ISS®) acquisition technology, the crosstalk is interference from other sources, and depends both on the time of initiation and distance separation. Often, the crosstalk can be severe due to the elimination of listening time as shown below in FIG. 2. Conventional sparse acquisition designs and seismic data processing algorithms do not, so far as is known, provide a capability to deblend the crosstalk interference below the signal level.
Broadband, high-channel count onshore seismic acquisition systems with the capability to quickly and efficiently acquire high-source and receiver density surveys offer new opportunities to improve seismic data quality. Acquisition systems with the capability of continuously recording very wide receiver super-spreads with time-distance rules for source initiation can implement both conventional flip-flop and slip-sweep productivity methods, and more recent high-productivity innovations including Distance-Separated-Simultaneous-Source (DS3), Distance-Separated-Simultaneous-Slip-Sweep (DS4) and Independent Simultaneous Sources (ISS®).
The choice of non-aggressive or aggressive time-distance rules depends on the ability of processing technology to suppress harmonic and crosstalk interference. The lowest risk of crosstalk and harmonic interference is achieved with flip-flop, DS3 or DS4 methods. Flip-flop acquisition is a ‘time-only’ rule with minimum time separation equal to the vibrator sweep plus recording listen time. DS3 acquisition is a ‘distance-only’ rule which allows one or more source fleets to start simultaneously as long as they are separated by some specified minimum distance such that crosstalk occurs outside the recording listen time. DS4 acquisition is a ‘time-and-distance’ rule which allows one or more source fleets to start simultaneously as long as they are all separated by some specified minimum distance and minimum slip-time.